The invention relates to control of laser parameters and material formulations to produce both light and dark laser markings and preselected one-color and multi-colored laser designs on plastic articles.
Laser marking is a well known and important means for quickly and cleanly inscribing plastic surfaces with identification marks, such as date codes, batch codes, bar codes or part numbers, functional marks, such as computer keyboard characters, and decorative marks, such as company logos. The most common laser marks are either a dark mark on a lighter colored background or a light mark on a dark colored background. However, colored laser marks on plastic articles, such as electronic components, exterior automotive parts, or utensils, and the like, are also desirable in order to eliminate the cost and environmental complications associated with inks, masks, and other printing or hot-stamping methods now employed for color imprinting.
The most common mechanism of laser marking of thermoplastic materials depends on the rapid production of heat in the irradiated portion of the plastic due to the absorption of the laser energy. Although some thermoplastics, such as polyethylene, polypropylene (PPRO) and polystyrene, are transparent to laser energy at certain wavelengths, they may be marked by including in the resin composition a laser energy-absorbing additive, such as carbon black, graphite, kaolin, mica, and the like, that increases the rate of temperature rise in the localized portion of the polymer exposed to the laser. Other polymers, such as polyvinylchloride, polyethylene terephthalate and acrylonitrile butadiene styrene (ABS) readily absorb laser energy and require little or no special additives.
A light, dark or colored laser mark on a thermoplastic material may be produced by several different mechanisms or combination of mechanisms, depending on the resin and additives employed, the nature of any colored pigments, and the laser energy characteristics. For example, using a pulsed TEA-CO.sub.2 (Transversal Excited Atmospheric Pressure carbon dioxide) laser, a dark marking on polyethylene containing an energy absorbing pigment (Afflair.RTM. Lustre Pigment, EM Industries, Hawthorne, N.Y.) can be produced at a relatively low energy level (3 joules/cm.sup.2) by heat-induced carbonization of the polymer and/or the pigment at the polymer matrix. A continuous wave (CW)-CO.sub.2 laser, however, does not produce sufficient energy to cause carbonization and the plastic melts without leaving a contrast mark. Polymers that have no or a low tendency to carbonize, such as polyolefins and high density polyethylene (HDPE), may show a light mark caused by foaming of the resin due to the heat produced by the laser energy, whereas other polymers, such as polycarbonate (PC), ABS and polystyrene, have a tendency to carbonize rather than foam. A light or a colored mark on a dark background may also be produced when a dark colored additive, such as carbon black or a dark color pigment, is combined with a resin and exposure to the laser results in vaporization or bleaching of the additive and exposure of an underlying heat-stable color pigment or dye or natural polymer color. A dark marking has also been achieved by the use of additives that are colorless in the visible light spectrum but which change into a visible dark or black product when irradiated by laser light just outside the visible range, such as by a Nd:YAG (Neodymium doped Yttrium Aluminum Garnet) laser (wavelength 1064 nm) or an excimer laser (wavelength 308 nm or 351 nm).
It has been desirable in the past to produce a laser mark with the highest visual contrast between the mark and the color of the surrounding plastic. Thus, the lightest (eg. whitest) or darkest (eg. blackest) marks or the highest contrast color marks have been those most strived for. High contrast dark marks have been achieved by a TEA-CO.sub.2 laser on laser markable thermosetting resins, such as an epoxy, phenol or bismaleimide resin, an unsaturated polyester resin or a urethane resin, by physically coupling a colorant that darkens at a temperature greater than 250.degree. C. to a laser energy absorbing additive, such as cordierite, crystalline zeolite, zirconium silicate and calcium silicate.
A light and a dark mark on the same polymer composition has also been reported. Using a TEA-CO.sub.2 laser and a composition comprising polyethylene, 1% of a mica-based additive and 0.01% of a red pigment, a white mark was achieved by bleaching of the red pigment at a low energy density (&lt;2.5 joules/cm.sup.2) and a black mark was achieved by carbonization of the polyethylene and the additive at a higher energy density (&gt;4 joules/cm.sup.2).
Color marks have been formed on a dark background by a Nd:YAG laser or a frequency doubled Nd:YAG laser (wavelength 532 nm), employing a polyacetal copolymer resin or a polybutylene terephthalate resin combined with a mineral black pigment (bone charcoal, bone black or ivory black) that is removed or destroyed by the laser, and a heat-stable organic and/or inorganic pigment or a polymer-soluble dye. Color marks have also been achieved with a Nd:YAG laser on thermoplastics that have been colored by an organic dye or pigment and an inorganic pigment of the same color, and which also contain carbon black. These color marks have the same color as the background color of the plastic, but have a lighter tone.
Although color laser-markable polymer compositions, such as those employing polyamides, polybutylene terephthalate, and acetal copolymers (eq., polyoxymethylene) and laser energy absorbing additives, are currently under development, the formulations are proprietary. It has been reported that, using these experimental formulations, black or white laser images have been formed on a blue polybutylene terephthalate and on a yellow acetal. Color-sensitive additives have also been incorporated into two different proprietary acetal formulations, such that when exposed to light from an Nd:YAG laser, the initial shades of the resins, dark hues of maroon, green, brown, and navy blue, become lighter (e.g, maroon to pinkish hue, navy blue to light blue) depending on the characteristics of the laser beam. A two-color mark has also been reported to be produced using a frequency-doubled Nd:YAG laser to form a green image on a proprietary yellow-pigmented polyamide.
Although a high contrast black, white or colored mark on polymer articles may be desirable in certain cases, there is a need for laser marking for wider applications, such as decorative multi-colored designs on plastic products, multi-colored company logos, and other distinctive multi-colored designs and logos that enhance a product's sales appeal. There is a further need for a process that provides multiple colored laser designs on a wide variety of different thermoplastics. Moreover, there is a need for a method of custom-designing the color of laser marks for each customer by preselection of the color hue(s) and for providing uniformity and reproducibility of the color of the marks from batch to batch. There is a further need for producing a plurality of different preselected specialty color markings on a single polymer article and incorporating computerized laser marking steps into the manufacturing process as the article moves down a production line.